Bushing



Patented Dec. 8, 1942 RUSHING Charles Philippe Boucher and Bertrand H. ll.

Noble, Paterson, N. I., aulgnors to Boucher Inl ventions, Ltd., Washington, D. C., a corporation of Delaware Application July 29, 1939, Serial No. 287,410

8 Claims. V(Cl. 174-153) Our invention relates to high potential bushings and more particularly concerns such bushings as may be used in connection with high potential electrical equipment, such as neon sign transformers.

One object of our invention, therefore, is the provision of a simple, economical and eillcient bushing assembly which is capable of withstanding high electrical potentials, which has great mechanical strength, and which is well adapted to withstand the varying conditions of actual practical use.

Another object is the provision of a compact and rugged high potential Abushing assembly which combines high internal dielectric strength with high resistance to external surface leakage, which affords protection of parts expensive to replace, and which permits ready replacement of parts likely to be damaged in handling or installing the associated equipment.

Another object is the provision of a high potential bushing assembly for electrical equipment in which detrimental flash-over, and possible destruction of insulation and damage to the equipment upon appearance of voltages in excess of rated voltagesis avoided in a simple, direct and fully reliable manner.

Other objects of our invention in part will be apparent and in part pointed out hereinafter.

The invention accordingly consists in the combination of elements, features of construction, and arrangement of parts, all as described herein, the scope of the application of which is indicated in the following claims.

In the drawing, the single figure is a longitudinal sectional view of a bushing assembly in accordance with`our invention,

As conducive to a clearer understanding of certain features of our invention, it may be noted at this point that in handling high potential electrical equipment such as neon sign transformers, and especially in the installation and repair of the same, a considerable breakage of bushings has been encountered. l

It is customary practice to assemble neon sign transformers, with their attendant enclosures and bushings, at. the place of manufacture, placing the transformers in their enclosures, mounting the bushings, and thereafter filling the enclosure with whatever insulated ller or compound it is desired to employ. Thus the transformers are shipped with their bushings in situ, and upon delivery, it is necessary only to connect the high tension terminal leads to the load. The bushings are of porcelain and considerable breakage has been encountered. In some cases the bushings are chipped in handling the equipment. In others, they are chipped or broken completely during the installation of theequipment, as by being struck with a wrench. Breakage in such cases requires costly repairs because the transformer must be torn down to remove the broken part from the casing of the equipment, and then rebuilt with a new bushing in place. Accordingly, an outstanding object of our invention is the production of a highpotential bushing assembly which is sturdy and not readily broken and in which external parts subject to mistreatment readily may be replaced when broken-all at small cost and without the necessity for tearing down the transformer.

In heretofore known bushing assemblies employing a plurality of parts, it has been customary to employ a clip to secure the several parts of the assembly together. This construction has been found to be not entirely satisfactory, however, for the reason that no adequate and lasting seal between the bushing and the casing of the electrical equipment, upon which the bushing is mounted, has been obtained. We attribute this to insuilicient pressure being obtained on the sealing gasket employed. An important object of our invention, therefore, is to eliminate this disadvantage of known multi-part bushing assemblies, and to provide a bushing assembly which is tightly and positively sealed to the equipment casing on which it is mounted.

Referring now more particularly to the practice of our invention, attention is directed to the drawing illustrating one embodiment of our invention. An inner bushing member l is provided which is formed of suitable material of high dielectric strength such as wet process porcelain. 'I'his possesses high mechanical strength. Moreover, such wet process porcelain is found to have an internal electrical resistance which is about seventeen times that of, say, the comparatively inexpensive dry process porcelain. Thus, despite the greater cost of the wet process porcelain, we prefer to employ this material in the production of the inner bushing member I, because it is so located that it must withstand the greatest electrical stress, as well as the various mechanical stresses encountered in use. Of course it is within the purview of our invention to substitute for the wet process porcelain from which the inner bushing member l is fashioned, some other suitable insulating material, either plastic or otherwise, having the desired high dielectric qualities and required mechanical strength,

Bushing member I is so shaped as to have a body portion 2 and a protruding extension 3. of reducedv section, extending to the right as seen in the drawing. While body portion 2 and the protruding extension I may be formed of any desired cross sectional shape, it is always desirable in high potential practice to eliminate wherever possible the occurrence of sharp bends or corners, in order to avoid local concentration of electro-static forces and attendant discharge. Accordingly, we prefer to shape the cross-section of parts 2 and 3 along some continuous curve, preferably making them circular. A 'I'he circular cross-section, and hence cylindrical elongated shape facilitates low manufacturing costs of these bushing assemblies. Ordinarily, in its manufacture, the bushing member is turned to desired size and shape.

The bushing member I is mounted interiorly of the casing of the associated electrical equipment with body part 2 abutting the inner surface of casing wall 4. A shoulder 3 is provided at the junction of body portion 2 and extension I, and in practice, the bushing I passes by its extension I from the interior of the enclosure through an opening I in the wall 4. This opening preferably is of such size as to loosely receive the extension 3i Moreover, shoulder 9 is nicely tted, to ensure intimate contact with the interior face of wall 4, and thereby effectively seal against leakage of the insulating compound with which the casing may be filled.

A conducting bolt passes centrally through bushing member I, by way of the longitudinal bore Ii provided in parts 2 and 3 of the said bushing I for this purpose. Bolt 5 is provided with a head portion I which is adapted to seat in a recess 8 in the bushing member body portion I. Bolt head and recess may be shaped to prevent twisting of the bolt within the bushing member. y

The inner bushing member I serves to insulate the casing against the high potential of the conducting bolt 5, and to this end, it is formed of the material of high dielectric qualities previously described. Moreover, it is formed of a material having high mechanical strength because in actual practical use it may be desired to lift or otherwise handle the transformer or other equipment by the bushing. The fit between the opening ID and the projecting extension 3 is made snug, to prevent dynamic shear resulting from play between these two parts when the equipment is lifted by means of the bushing.

To ensure that bushing member I properly insulates against the rated voltage of the equipment, the surface path II, I2, I3, I4 is of such length as to prevent high potential current leakage from bolt 5 to the wall 4. That is to say, the recess 8 of bushing I is of such depth as to ensure a leakage path of desired length. Furthermore, the thickness through the body portion 2 is suiiicient to prevent rupture across the bushing under the electrical stresses maintaining.

The length I5, I6 of the projecting extension of the bushing member is designed so that at rated voltage no sparking occurs between bolt 5 and the wall 4, either through the air or over the surface of the projecting extension. The annular section of the projecting extension 3 is of sufficient thickness to ensure against rupture thereacross, either from electrical stresses, or rough mechanical handling.

The conducting bolt 5 extends axially through the bushing member I to the exterior, beyond the wall 4 and beyond the projecting extension I, in a threaded portion I1. This bolt is connected at its head 1 to a conductor I3 from the transformer or other high potential electrical equipment housed within casing 4. Conductor Il is secured to the head 1 in any suitable manner, as by soldering, or by looping it under the head. Bolt l is thus seen to conduct the high potential current either into or out of the enclosure, as desired, and at the same time, as will be more fully pointed out hereinafter, it is used to hold tightly together the several parts oi' the bushing assembly to give a strong and rugged construction suited to the varying conditions of actual use.

In order to protect bushing member I from breakage and from weathering and the formation of a conductive film along its surface, a covering body I9 is provided. The body or outer bushing member is recessed axially at 2l to receive the projecting extension of bushing member I with a fair degree of snugness. Conveniently, this recess 20 is continued as a bore 2I of smaller diameter, to receive the bolt 5, the threaded end II of which protrudes to the exterior therebeyond. It will thus be seen that the covering body I9, when in its assembled position, lies axially with respect to the bushing member I. Moreover, it will be seen that it lies on the opposite side of the casing wall 4 from the body portion 2, and completely encloses the projecting extension 3.

While the outer bushing member or covering body I9 may be formed of any desired insulating material, it advantageously is shaped from an inexpensive material of low dielectric and mechanical strength as compared with the wet process porcelain from which we prefer to form the inner bushing member. For example, as pointed out hereinbefore, it is desired that the inner bushing member of high dielectric strength be protected from breaking and chipping in handling and installation of the equipment by an outer bushing member which lies entirely outside of the equipment casing and which is formed of inexpensive material and of a shape which is conducive to low production costs. Moreover, it is unnecessary that this element have the high dielectric strength of the inner bushing member I because the region of the conducting bolt 5 passing through the covering body I9 is so much more remote from the casing wall 4 than in the case of the inner bushing member. All that is required of the covering body I9 is that its surface be glazed, vitrifled, or in general, non-moisture absorbing, and that its effective surface length be suflicient to prevent surface leakage from bolt 5 to wall 4. In order to increase the effective length of surface, to the aforementioned end, the covering body is provided with suitable profile irregularities, such as the skirted portion 22.

Outer bushing member or covering body Il is recessed centrally as at 23, the recess terminating in a shoulder 24. This shoulder is provided to receive a washer 25, and a nut 25, threadedly engaging the threaded extension I'I of bolt 5. Thus, when the nut is turned down on bolt 5, the outer bushing member I9 is drawn tightly against the wall 4 thru the action of washer 25. Interposed between the wall 4 and the inner face of the outer bushing member, we prefer to insert a sealing gasket 21 of any suitable material. This gasket is compressed when the outer bushing member is drawn into place, and

serves to prevent any moisture from entering the transformer enclosure, through breathing. Moreover, this gasket also cushions the outer bushing member I9 to prevent the latter from cracking under local compressive strains.

The space 28 sealed off by the washer 25 and gasket 21 and defined by bolt 5, outer bushing member I9 and the projecting portion of inner bushing 3, may be conveniently iilled with a viscous insulating material, such as a thick insulating varnish or wax, or an` insulating oil, to make the bushing assembly airand moisturetight. Whenever this insulating material is use d washer 25 is in the form of a steel lock-washer which allows for the expansion and contraction of the bushing assembly in use. In another embodiment of our invention We fashion Washer 25 of senil-hard, pliable material, such as lead, which yields and compresses under the pressure of the nut 26 and obstructs the space 28, so that the insulating filler may be omitted, if desired.

To prevent the outer bushing member I9 from moving relative to casing wall 4, one of these elements is provided with a notch and the other with a cooperating protuberance. Conveniently, the wall contacting face of outer bushing member I9 is provided with a notch 29 in which is received, when the bushing is assembled, one face of a struck portion 21A in the gasket 2l. Extending from the wall 4, a protuberance 30 conforms to the other face of the struck portion 21A of the gasket 21. In this simple manner the outer bushing member is protected against axial rotation.

A high-potential conductor (not shown) is con nected to our bushing assembly by way of bolt and a second nut 3I threadedly engaging the bolt. When desired, a washer 32 is used to separate nuts 25 and 3I in order to prevent a loosening of the former in backing off the latter while changing the high-potential conductor.

Where there is any likelihood of a person or object contacting the bolt 5 or the high-potential connection to the bolt, or Where there is possibility of damage thereto, it is desirable to protect the end of the bolt, and the connecting end of the high-potential conductor, in some suitable manner. To this end, we provide a cover 33 of soft, pliable material, such as rubber or a suitable plastic having the elastic qualities or characteristics of rubber. One end, 34, of this cover protrudes in the manner of a cap, and this end is illustrated as provided with a blind hole 35 which fits over the end of bolt 5. We prefer to form the cover so that it engages the bolt by tight friction fit. Once pressed into position, the cover 33 will remain in place. A notch 36 is provided near the inner end of the cover 33, to house snugly, and to permit the passage therethrough, of the high potential cable.

It will be noted that at its inner end, the cover member has outwardly aring or tapered, thinned knife-edges 31, also formed of the soft, pliable rubber or the like, to adapt the cover sensitively to the contours of the outermost edge of the outer bushing member I9. Thus, when the cover 33 is pressed onto the end of bolt 5, the knife-edges 31 spread on the surface of the covering body' I9 and provide a perfectly dust-proof and bug-proof seal. Moreover, this seal to a large extent prevents corrosion or other deterioration of or damage to the bolt itself, or to the connection.

In many instances, it is desirable to prevent occurrence of over-voltages, transient or otherwise, from locally overstraining any part of the electrical equipment or system with which our bushing assembly is associated. To this end, one phase Ans of our invention is directed to the utilization, if desired, of what we call a set-gap corona discharger. As shown in the drawing, this corona discharger comprises opposed washers 38 and 39` of conducting material mounted axially in the assembly. Washer 38A is disposed about the base of the projecting extension 3 of the inner bushing member and against the shoulder 9 of body portion 2. This washer has an outwardly extending flange 40 in electrical contact` with casing wall 4, and an axially extending rim or ange 4 I. Facing the washer 38 there is the washer 39 mounted about bolt 5, in electrical contact therewith, and seated on the extreme end of the projecting extension 3 of the inner bushing member. Washer 39 is provided with an axially turned peripheral flange 42. 'I'he iiange 4I of washer 38 and ilange 42 of washer 39 are directed towards each other, and

together comprise the operating part of the corona discharger. The space between those two rims constitutes the spark-gap of the discharger, and this space will break down uniformly, in a brush discharge, upon critical voltages being impressed upon the equipment. The brush, or corona, discharge takes place between the washer 39, at high potential, and the washer 38 at ground potential, and so long as the excessive potential is present this uniform, tuft-like discharge continues, thereby precluding a disruptive spark discharge within the equipment.

To facilitate corona discharge action, and for general purposes of safety, the wall 4 or other part of the transformer casing may be grounded, in conventional manner, as shown at 43.

When a corona discharger is employed, it may be desirable t'o normalize pressure conditions within the bushing assembly, and to this end, gases stressed as by increased pressure resulting from corona discharge may be vented, if desiredf' through opening's 44 formed in the outer bushing member I9. Where this expedient is adoptedfit of course is necessary either to omit theuse of the seal of viscous insulating material for the space 28, or `to confine the seal to the space between Washers 25 and 39, and bolt 5 and outer bushing member I9.

It will readily be seen, from a consideration of the foregoing, that there is provided in our invention a bushing assembly in which the various objects hereinbefore noted, together with many practical advantages,are fully achieved. Our new bushing assembly is much more dependable than the assemblies hitherto employed by those working in the art. 'I'he bushing assembly enjoys a combination of high dielectric strength with high resistance to external surface leakage. It is strong and rugged in construction. 'I'he outer bushing member, or covering body, protects the projecting extension of the inner bushing member from breakage or damage. Where the outer bushing member is chipped or otherwise damaged in handling the equipment with which the bushing is associated, it can be readily replaced without any necessity for opening the equipment closure, or impairing in any way the function of the same.

As many possible embodiments may be made of our invention, and as many changes may be made in the embodiment hereinbefore set forth, it will be understood that all matter described herein, or shown in the accompanying drawing, is to be interpreted as illustrative, and not in a limiting sense.

We claim:

1. A high potential bushing assembly, combining high internal dielectric strength with high resistance to external surface leakage, and adapted to be mounted on the casing of high potential electrical equipment, said bushing assembly comprising, in combination, an elongated, Wet process porcelain bushing member, having a shoulder portion thereof and an axial bore extending therethrough and being adapted to bear snugly against an inner surface ofthe casing of the equipment and protrude exteriorly therethrough; a conducting terminal bolt for said equipment extending through said bushing member and protruding exteriorly in a threaded extension beyond the same; a dry, molded process porcelain second bushing member skirted to increase the effective path of surface leakage, having an axial bore receiving said bolt and enclosing the protruding portion of said hrstmentioned bushing member and adapted to bear against an outer surface of said casing; and means engaging the threaded portion of said terminal bolt for maintaining said bushing members in proper abutting relation to said equipment casing.

2. A high potential bushing assembly, combining high internal dielectric strength with high resistance to external surface leakage, said bushing assembly comprising, in combination, a wet process porcelain bushing member of high dielectric and mechanical strengths having an elongated portion and having an axial bore extending therethrough, a conducting terminal bolt extending through said bushing member and protruding exteriorly in an extension beyond the same, a dry process porcelain bushing member of high resistance to external surface leakage having an axial bore receiving said bolt and enclosing the Wet process porcelain bushing member, means engaging the extending portion of said terminal bolt and maintaining said bushing members in assembled relationship; means for securing a high-tension cable to the extension of said bolt; and an insulating cover engaging the free end of the bolt to close the same substantially completely against the exterior.

3. A high potential bushing assembly, combining high internal dielectric strength with high resistance to external surface leakage, and adapted to be mounted on -the casing of high potential electrical equipment, said bushing assembly comprising, in combination, an elongated, wet process porcelain bushing member, having a shoulder portion thereof and an axial bore extending therethrough and being adapted to bear snugly against an inner surface of the casing of the equipment and protrude exteriorly therethrough; a conducting terminal bolt for said equipment extending through said bushing member and protruding exteriorly in a threaded extension beyond the same; a dry, molded process porcelain second bushing member skirted to increase the effective path of surface leakage, having a recessed end-portion and an axial bore receiving said bolt and enclosing the protruding portion of said first-mentioned bushing member and adapted to bear against an outer surface of said casing; means seated in the recessed endportion of said second bushing member and engaging the threaded portion of said terminal bolt for maintaining said bushing members in proper abutting relation to said equipment casing; means for securing a high tension cable to the threaded extension of said bolt; and a cap of soft, pliable insulating material engaging the free end of the bolt by tight frictional ht to close the same substantially completely against the exterior, the open ene` of said cap having outwardly haring edges which engage against the outermost edge oi' said second bushing member, thus providing a dust-prooi and bug-proof seal for the cable connection.

4. A high potential bushing assembly, combining high internal dielectric strength with high resistance to external surface leakage, and adapted to be mounted on the casing of high potential electrical equipment, said bushing assembly comprising, in combination, an elongated bushing member of high dielectric and mechanical strengths having a shoulder portion thereof and an axial bore extending therethrough and being adapted to bear snugly against an inner surface of the casing of the equipment and protrude exteriorly therethrough; a conducting terminal bolt for said equipment extending through said bushing member and protruding exteriorly in a threaded extension beyond the same; a second bushing member of high resistance to external surface leakage, having an axial bore receiving said bolt and enclosing the protruding portion of said hrst-mentioned casing; means engaging the threaded portion of said terminal bolt for maintaining said bushing members in proper abutting relation to said equipment casing; and corona discharge means for said equipment comprising two opposed hanged washers, disposed coaxially on the protruding extension of said mst-mentioned bushing member, with hanged portions facing each other to determine the maximum potential for the equipment, one said hanged washer being seated at the base oi' the protruding extension of said member and in electrical contact with the equipment casing, and the other being seated at the free end of the protruding extension and in electrical contact with said terminal bolt.

5. A high potential bushing assembly, combining high internal dielectric strength with high resistance to external surface leakage, and adapted to be mounted on the casing of high potential electrical equipment, said bushing assembly comprising, in combination, an elongated bushing member of high dielectric and mechanical strengths having a shoulder portion thereof and an axial bore extending therethrough and being adapted to bear snugly against an inner surface of the casing of the equipment and protrude exteriorly therethrough; a conducting terminal bolt for said equipment extending through said bushing member and protruding exteriorly in a threaded extension beyond the same; a second bushing member of high resistance to external surface leakage, having an axial bore receiving said bolt and enclosing the protruding portion of said hrst-mentioned casing; means engaging the threaded portion of said terminal bolt for maintaining said bushing members in proper abutting relation to said equipment casing; corona discharge means for said equipment comprising two opposed flanged washers, disposed coaxially on the protruding extension of said first-mentioned bushing, with hanged portions facing each other to determine the maximum potential for the equipment. one said hanged washer being seated at the base oi' the protruding extension of said member 'and in electrical contact with the equipment casing, and the other being seated at the free end of the protruding extension and in electrical contact with said terminal bolt; and means comprising breathing holes in said second bushing member for relieving the pressure of expanding atmosphere between the space of said first and second bushing members during operation of said corona discharge means.

6. A high potential bushing assembly, combining high internal dielectric strength with high resistance to external surface leakage, and adapted to be mounted on the casing of high potential electrical equipment, said bushing as'- sembly comprising, in combination, a Wet process porcelain bushing member of high internal dielectric and mechanical strengths having a shoulder portion thereof and an axial bore extending therethrough and being adapted to bear snugly against an inner surface of the casing of the equipment with a portion protruding exteriorly therethrough; a conducting terminal bolt for said equipment extending through said bushing member and protruding exteriorly beyond the same; a dry process porcelain bushing member of high resistance to external surface leakage, having an axial bore sealed from the exterior receiving said bolt and enclosing the protruding portion of said wet process porcelain bushing lmember, and being in locked engagement with an outer portion of said equipment casing; and tightening means on said bolt for maintaining said bushing members and casing in abutting relationship.

7. A high potential bushing assembly, combining high internal dielectric strength with high resistance ,to external surface leakage, and adapted to be mounted on the casing of high potential electrical equipment, said bushing assembly comprising in combination, a wet process porcelain bushing member o1' high internal dielectric and mechanical strengths having a shoulder portion thereof and an axial bore extending therethrough and being adapted to bear snugly against an inner surface of the casing of the equipment with a portion protruding exteriorly therethrough; a conducting terminal bolt for said equipment extending through said bushing member and protruding exteriorly in an extension beyond the same; a dry process porcelain bushing member of high resistance to external surface leakage, having an axial bore sealed from the exterior receiving said bolt and enclosing the protruding portion of said wet process porcelain bushing member; means maintaining the said bushing members in assembled relation, with the dry bushing member in locked engagement with said equipment casing; and an insulating cap engaging the free end of the bolt to close the same substantially completely against the exterior.

8. A high potential bushing assembly, combining high internal dielectric strength with high resistance to external surface leakage, comprising, in combination, a Wet process porcelain bushing member having an elongated portion and having an axial bore extending therethrough, a dry process porcelain bushing member having an axial bore and receiving said elongated portion of said wet process porcelain bushing member, and axial conductor means positioned within the bores of said bushing members and maintaining the same in assembled relationship.

CHARLES PHILIPPE BOUCHER. BERTRAND H. H. NOBLE. 

